The field of this invention relates to the liquid-phase oxidation of pseudocumene (PSC). In one aspect, this invention relates to conducting the initial part of the reaction in a semi-continuous or batch mode followed by a batch tail-out wherein most of the bromine promoter and cerium in the plus three valence is added in the batch tail-out stage, thus reducing the contact time of the polycarboxylic acid moieties with cobalt-manganese-bromine or zirconium-cobalt-manganese-bromine catalysts and improving the yield of trimellitic acid (TMLA) from PSC.
The bromine-polyvalent metal catalysts in acetic acid solvent have been in commercial use in many countries for the manufacture of terephthalic acid from p-xylene for many years. However, in the absence of acetic acid solvent, the best yield of a single phthalic acid (e.g., terephthalic acid) on a once-through basis of the xylene amounted to about 20 weight percent (12.8 mole %), according to U.S. Pat. No. 2,833,816. According to U.S. Pat. No. 3,920,735, the Mn-Br and Co-Mn-Br catalyst systems are improved by the addition of zirconium. However, not mentioned, but illustrated in Tables I, II and IV in U.S. Pat. No. 3,920,735, is the fact that, when part of the zirconium is added, combustion of the feedstock to carbon dioxide increases. The use of cerium has been disclosed in the U.S. Pat. No. 3,491,144 however, in that reference cerium is not added to the batch tail-out part of the reaction in the plus three valence state wherein the amount of bromine added is reduced.